The present invention relates to a method and a device for data extraction from a data stream containing at least one data packet, in particular for use in a receiver of a wireless digital communication system.
In TDMA based wireless digital communication systems, like Bluetooth, data is transmitted over an air interface in separate packets with well-defined structure. Data is transmitted in a data stream containing a plurality of data packets. Before a receiver can extract the wanted data out of these packets, it has to perform different processing steps.
Signal parameters have to be estimated and appropriate compensation for them has to be done. A synchronization unit, that will be described below, is responsible for the initialization and data extraction in a Bluetooth receiver. Bluetooth is a global standard for wireless, short-range, low-power, low-cost data communication. On the physical layer a binary, gaussian shaped frequency modulation with low modulation index is used.
FIG. 1 shows an example of the receiving chain of a wireless TDMA receiver. After down-conversion of the carrier frequency to an intermediate frequency IF by a mixer 10, channel selection is done with a channel filter 11. Then after demodulating the waveform in a demodulator 12, the waveform (shown In FIG. 2 (a) as wavy line W) is transferred into a digital representation (shown in FIG. 2 (a) as X-es DW) by an AD converter 13. As a consequence a synchronization unit 14, that receives the digital representation of the received waveform, that is the digitized demodulated waveform of the received signal at its input, can be fully digital. At its output, that is connected to a link controller 15, it provides the extracted bits of the received data packets for further processing on higher layers.
In order to enable data extraction different tasks have to be performed by the synchronization unit 14.
The bits of a data packet are transmitted over an air Interface 9, that is represented by an antenna in FIG. 1, in a sequence of binary FM-modulated symbols. Thus, each symbol represents a received bit. After the FM-demodulation, the synchronization unit 14 has to process these symbols as the symbols arrive at the synchronization unit 14 with an unknown timing and unknown DC level.
However, for data extraction unknown parameters have to be estimated. The DC level resulting from TX and LO offsets as well as from demodulator 12 deviations can be a few times the amplitude of the waveform itself. Under certain conditions, for example the first connection setup, the timing is totally unknown. For successive packets there is at least a timing uncertainty of ±10 symbols.
Before data extraction can be started an appropriate initialization for the applied algorithms is required.
First of all the presence of a packet has to be detected. When a packet has been detected, the accurate symbol timing needs to be determined. Also a DC estimate needs to be available when data extraction starts.
Data extraction requires that for each symbol a 1/0 or a bit decision is made, that is it has to be decided whether a symbol represents 1 or 0. A common method for this decision is to slice the waveform, that is the sequence of symbols with a DC estimate (shown as line DC In FIG. 2 (a)) into decided but oversampled bits illustrated In FIG. 2 (b). That is, based on the DC estimate for each of the samples within one symbol, a separate bit decision is made. Then for each symbol one sample out of the oversampled bit stream is taken as the result of the actual 1/0 decision. For an optimum bit error rate performance under noisy conditions, the samples corresponding to the symbol phase with optimum signal to noise ratio (S/N) (—the so called golden samples—) have to be taken. FIG. 2 (c) shows the golden samples taken from the oversampled bit stream by appropriate sampling and the corresponding bits.
A DC estimate can be obtained by freezing the output of a low pass filter at an appropriate time Instant when a packet has been detected. The position of a golden sample is assumed to be in the symbol center and therefore can be directly derived from the symbol timing.
When data extraction has been started DC estimate and symbol timing estimate may be continuously corrected.
Further, in some known solutions a synchronization unit processes only already sliced bits, that is it receives the oversampled bit stream. In this case, DC estimation is done on the radio frequency RF side. However, different DC estimation methods (fast and slow adapting low pass filters) should be used for packet search and data extraction due to different performance requirements. Bit slicing after a false alarm (with a slow adapting low pass filter) is not necessarily correct and a valid packet may be missed therefore.
Consequently, receivers have to perform a tradeoff between the packet loss rate (packet not recognized) and the false alarm probability (erroneous indication of a packet). Some receivers simply switch from an unsynchronous scanning mode into a synchronous mode in case a packet has been detected. In case of a false alarm the detection of a valid packet is often blocked then until the false alarm is recognized on higher layers.